A fistula is an abnormal passage typically between two organs, such as an artery and a vein. An arterio-venous (AV) fistula is a natural or an artificial graft, typically made of ePTFE (expanded PTFE), between a vein and an artery. An AV fistula, as used herein, also includes naturally-occurring native tissue tubular connections between a vein and an artery. AV fistulas are often used to provide multiple needle access sites for hemodialysis. The AV fistula also helps to increase blood flow through the vein to accommodate the flow rate of blood needed for hemodialysis.
One problem associated with AV fistulas is the progressive narrowing of the AV fistula at the junction with the vein. Such obstructions occur when vascular muscle cells begin growing inwardly causing, for example, thrombosis within the AV fistula. When the thrombus becomes sufficiently large, blood flow decreases and the AV fistula ceases to be effective. It has been found that graft patency after six months is only 66% and that graft failure occurs, on the average, after 18 months.
Improved graft patency has been achieved by the use of vascular clips instead of suturing the AV fistula to the vein. Variations in the angle of implantation have also been shown to affect AV fistula patency. The use of a short length of a PTFE graft has been inserted in the vein to improve patency. (A. S. Coulson, et al., A Combination of the Elephant Trunk Anastomosis Technique and Vascular Clips for Dialysis Grafts, Surgical Rounds, 596–608, November 1999.) Also, a PTFE bypass graft to a proximal dilated vein has been used in response to the occurrence of graft-vein stenosis. (Polo, J. R., The State of the Art of Surgical Treatment for Failing Grafts, The Seventh Biannual Symposium on Dialysis Access—Vascular Access for Hemodialysis VII, pp.8–9, May 2000.) Balloon angioplasty and endovascular stents may be used to treat stenosis in AV fistulas (J. E. Aruny, et al., Quality Improvement Guidelines for Percutaneous Management of the Thrombosed on Dysfunctional Dialysis Access, JVIR, 10:491–498, April 1999.) However, there still exists the need to stop, or at least slow, the obstruction of the AV fistula to prolong the patency of the graft.
An endoluminal prosthesis typically comprises at least one radially expansible, usually cylindrical, body segment. By “radially expansible,” it is meant that the body segment can be converted from a small diameter configuration (used for endoluminal placement) to a radially expanded, usually cylindrical, configuration which is achieved when the prosthesis is implanted at the desired target site. The prosthesis may be non-resilient, e.g., malleable, thus requiring the application of an internal force to expand it at the target site. Typically, the expansive force can be provided by a balloon catheter, such as an angioplasty balloon for vascular procedures. Alternatively, the prosthesis can be self-expanding. Such self-expanding structures may be provided by a temperature-sensitive superelastic material, such as Nitinol, which naturally assumes a radially expanded condition once an appropriate temperature has been reached. The appropriate temperature can be, for example, a temperature slightly below normal body temperature; if the appropriate temperature is above normal body temperature, some method of heating the structure must be used. Another type of self-expanding structure uses resilient material, such as a stainless steel or superelastic alloy, and forming the body segment so that it possesses its desired, radially-expanded diameter when it is unconstrained, e.g., released from radially constraining forces of a sheath. To remain anchored in the body lumen, the prosthesis will remain partially constrained by the lumen. The self-expanding prosthesis can be delivered in its radially constrained configuration, e.g. by placing the prosthesis within a delivery sheath or tube and retracting the sheath at the target site. Such general aspects of construction and delivery modalities are well-known in the art.
One type of endoluminal prosthesis includes both a stent component and a graft-type covering component. These endoluminal prostheses are often called stent grafts. A stent graft is typically introduced using a catheter with both the stent and graft in contracted, reduced-diameter states. Once at the target site, the stent and graft are expanded. After expansion, the catheter is withdrawn from the vessel leaving the stent graft at the target site. Grafts may be made of, for example, PTFE, ePTFE or Dacron® polyester.
It has been found effective to introduce pores into the walls of the graft to provide in growth of tissue onto the walls of the graft. With larger diameter grafts, woven graft material is often used. In small and large diameter vessels, porous fluoropolymers, such as ePTFE, have been found useful.
Coil-type stents can be wound about the catheter shaft in torqued compression for deployment. The coil-type stent can be maintained in this torqued compression condition by securing the ends of the coil-type stent in position on a catheter shaft. The ends are released by, for example, pulling on wires once at the target site. See, for example, U.S. Pat. Nos. 5,372,600 and 5,476,505. Alternatively, the endoluminal prosthesis can be maintained in its reduced-diameter condition by a sleeve; the sleeve can be selectively retracted to release the prosthesis. A third approach is the most common. A balloon is used to expand the prosthesis at the target site. The stent is typically extended past its elastic limit so that it remains in its expanded state after the balloon is deflated and removed. One balloon expandable stent is the Palmaz-Schatz stent available from the Cordis Division of Johnson & Johnson. Stents are also available from Medtronic AVE of Santa Rosa, Calif. and Guidant Corporation of Indianapolis, Ind.